Microbe(s)-on-microbe action

Poor old Vibrio vulnificus, it just can’t catch a break. This Gram-negative marine bacterium (and occasional human pathogen) is the first species found to be infected by both a virus and a predatory bacterium.

Let’s step back a moment and look at the smaller picture. Just like humans, bacteria are regularly infected by viruses. These viruses are known as bacteriophages, and they are the most abundant and diverse organisms on Earth: nobody really knows how many exist, but estimates suggest that there might be as many as 1031 on the planet (that’s more viruses than there are stars in the universe). Sometimes, after infection, the viruses integrate their genetic material into the host bacterial chromosome but remain dormant; other times, they force the bacteria to make multiple copies of themselves until the host cell can’t take any more and explodes.

As well as being infected by viruses, bacteria can be colonised by other bacteria. This can be for mutual benefit (as we’ve written about previously), but frequently the infection kills the infected cell.

Given that death of the host cell is a common outcome of both viral and bacterial infection, this new discovery – published in mBio – is very odd indeed. The research involved mixing V. vulnificus with equal numbers of the bacteriophage CK2 and predatory bacteria of the group ‘Bdellovibrio and like organisms’ (BALOs).

Electron microscopy of the mixture revealed V. vulnificus cells to be infected by CK2 or BALOs or, in some cases, both. This is strange because both the virus and the BALOs require an intact host cell in which to multiply or feed. An infection by one is very likely to upset the delicate internal balance of the V. vulnificus cell, which should prevent infection by the other.

So why is it happening? The authors of the paper made a few suggestions. Perhaps this is all just a fluke – maybe the virus infected first, and the BALO infected shortly afterwards. Perhaps there is some advantage to the dual infection – both the virus and the BALO get a safe haven to hide in during unfavourable conditions, while they compete for resources.

There are clearly many questions to be answered. How often does this happen in nature? Does it happen at all? What impact does it have on the microbial environment? This new discovery has opened up a new avenue for research, but spare a thought for V. vulnificus — its loss is our gain.